Re: [Haskell-beginners] Operator orders affect parse result

According to the given code, that result seems to be fine... Apply / to a and be and that result is applied to * together with c. Only odd thing is that you named the / Multiply and the * Divide. Am 13.04.2015 09:50 schrieb "m00nlight" :

Hi Francesco,

Thanks for your reply. But after I change the aOperators like the following, it still produce non-correct parse result.

aOperators = [ [Prefix (reservedOp "-" >> return (Neg ))] , [ Infix (reservedOp "*" >> return (ABinary Divide )) AssocLeft, Infix (reservedOp "/" >> return (ABinary Multiply )) AssocLeft] , [Infix (reservedOp "+" >> return (ABinary Add )) AssocLeft, Infix (reservedOp "-" >> return (ABinary Subtract )) AssocLeft] ]

parse result:

*Main> parseString "res := a / b * c" Assign "res" (ABinary Divide (ABinary Multiply (Var "a") (Var "b")) (Var "c")) *Main>

--m00nlight

在2015年04月13 15时21分, "Francesco Ariis"写道:

On Mon, Apr 13, 2015 at 02:56:47PM +0800, m00nlight wrote:

...

Dear Haskellers,

I have tried the "while language parser" tutorial on haskellwiki, but I encounter an problem I can not understand here, The following is the code:

http://pastebin.com/saC9vwCn

So it seems that the order of the operator will affect the parse result. What's the problem with my code? How can I make my program to produce consistent parse result.

Checking the documentation for OperatorTable [1] (the first argument to buildExpressionParser), it seems it is working as expected:

An OperatorTable s u m a is a list of Operator s u m a lists. The list is ordered in descending precedence. All operators in one list have the same precedence (but may have a different associativity).

I think you can take advantage of the fact that OperatorTable is /a list of lists/, hence

aOperators = [ [Prefix (reservedOp "-" >> return (Neg ))] , [Infix (reservedOp "/" >> return (ABinary Divide )) AssocLeft, Infix (reservedOp "*" >> return (ABinary Multiply )) AssocLeft] , [Infix (reservedOp "+" >> return (ABinary Add )) AssocLeft] , [Infix (reservedOp "-" >> return (ABinary Subtract )) AssocLeft] ]

should do the trick (try swapping the elements in the second list. Will this do?

[1] http://hackage.haskell.org/package/parsec-3.0.0/docs/Text-Parsec-Expr.html#t... _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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